A conventional motor includes a rotor carrying magnets which induce a voltage at the terminals of one or a plurality of coils, and these motors have characteristics which are very interesting for applications to timepieces. In fact, the speed of rotation of the motor can be synchronized with a given reference frequency, for example, from a quartz oscillator connected to a frequency divider, by means of a regulating circuit which proportions the electrical energy furnished to the motor.
This poses few problems in applications where the energy is not limited. In contrast, in quartz watches where the utilization of a complementary MOS logic is practically indispensable, the utilization of analog circuits or of high pass or low pass filters is practically excluded, and the principal difficulty resides in the fact that the speed of rotation of the motor is very variable and thereby one can not utilize a simple phase comparator because this would risk synchronizing on a sub-multiple of the desired speed or of desynchronizing at the least perturbation due to accelerations of the support.